In magnetic resonance imaging and spectroscopy, magnetic fields are used to manipulate nuclear magnetic resonance signals. Typically, time-varying magnetic gradient fields in multiple directions are superimposed to a constant, homogeneous main magnetic field to create spatial phase modulations in the magnetization of the object under examination. The homogeneous field is usually generated by a resistive or superconductive electromagnet. The gradient fields are usually generated by applying specifically shaped current waveforms to a multiple of gradient coils.
For accurate MR imaging or spectroscopy it is important that the main magnetic field and the gradient fields follow precisely the desired spatial and temporal patterns. However, in practice electromagnetic coupling among the various coils as well as between the coils and other components of the apparatus distorts the ideal field patterns and time courses. Timing imperfections in control electronics have similar effects. Spatial and temporal variations of the magnetic field can also arise from temperature changes in the apparatus, which frequently occur during operation. Another source of field variation is the magnetic susceptibility of the subject or object under examination. It causes added field contributions, which can vary over time, e.g. due to physiological motion like breathing. MR images and spectra acquired in the presence of such field deviations typical exhibit errors, distortions, artifacts, or signal losses.
For instance in U.S. Pat. No. 6,294,916 a magnetic resonance imaging apparatus is described, which uses magnetic field gradients X, Y, Z to spatially encode the magnetic resonance signals arising from a patient on a couch in the bore of a main magnet. Thermal stresses arising from aggressive gradients during multiple acquisitions result in imperfectly repeated gradients and resulting image artefacts. A probe comprising an MR active substance is used, equipped with a gradient coil set similar to that for imaging, and is fed by currents derived from the imaging gradient coils, connected so as to produce an opposing gradient surrounding an MR active substance in the probe. The probe produces a signal to be used to monitor the gradient, while overcoming de-phasing in the active substance.
The construction of the probe with the additional gradient coils of above mentioned apparatus is quite complicated and technically difficult to realize. The additional gradient coils will generate magnetic fields also outside the probe and couple with any magnetic or inductive structural elements of the apparatus. As a consequence, the additional gradient coils are likely to interfere with the actual experiment as well as with potential further probes. Moreover, coupling to the main gradient coils will cause error currents in the additional gradient coils, hence limiting the ability to prevent dephasing.